Various front-mounting conversion lenses (so-called wide converters) have been proposed. Such a conversion lens is attached to an object side position, that is, a position in front of an image pickup lens of an image pickup apparatus such as a still camera or a video camera and configured to change the focal length of the entire image pickup system toward the wide angle side while the focal plane of the entire system is maintained at a certain position.
For example, Japanese Unexamined Patent Application Publication No. 2002-214529 describes a wide-angle conversion lens that is attached to an object side position of a zoom lens, which is a main lens system, and can increase the view angle of the entire lens system. This wide-angle conversion lens is constituted by three lenses: a concave meniscus lens whose convex surface is positioned to face the object side; a plastic aspheric lens; and a convex lens disposed at a position closer to the image side than the position of the concave meniscus lens.